1. Field of the Invention
The present invention relates to a power supply control circuit, power supply and power supply control method for converting an existing power supply at a first voltage and power rating to a secondary voltage and power rating.
2. Description of the Related Art
In a portable electronic device, such as a notebook size personal computer, an AC adapter or automobile battery adapter, etc. can be used for an external power supply. The automobile battery adapter is called a power supply. The power supply provides an output power by adjusting the power from the car battery to the power required for the portable electronic device.
The capacity of the power supply and the AC adapter generally determine the maximum output voltage and the maximum output current. This maximum output voltage and the maximum output current are defined as the rated output. The power supply always operates to compensate to the rated output even if the input power varies. Therefore, when an input voltage is high, an input current is small. On the other hand, when an input voltage is low, an input current becomes large.
FIG. 1 is a schematic diagram of a power supply circuit of the related art. The circuit includes a noise eliminating filter section 10, a voltage converting section 20 for converting an input power to an output power, a rectifying section 30 for rectifying an output of the secondary side, an output detecting section 40 for monitoring an output of the secondary side and a coupler 50 for transmitting the condition of an output detecting circuit in the secondary side to the voltage converting circuit in the primary side.
The filter section 10 is formed of a coil L1 and a capacitor C1. The filter section 10 is a circuit for preventing propagation of noise generated in the voltage converting section 20 to the input side.
The voltage converting section 20 includes a transformer T1 for voltage conversion, a transistor Tr1 for shutting off a current flowing through the transformer T1 and a control circuit 60 for controlling the transistor Tr1.
The rectifying section 30 includes a rectifying diode D1 for rectifying a current outputted from the voltage converting section 20 and a capacitor C2 for smoothing the rectified current.
The output detecting section 40 includes a sense resistor R0 for detecting an output current value of the power supply and a sense circuit 70 for detecting a voltage value across both ends of the sense resistor R0.
The coupler 50 is a circuit for transmitting an output of the sense circuit 70 to the control circuit 60. In the coupler 50, a photo-coupler is used in general to electrically insulate the primary side and secondary side.
In FIG. 1, when the transistor Tr1 is ON, an input current flows in the primary side coil of the transformer T1. When the transistor Tr1 is OFF, an output current flows in the secondary side of the transformer T1. The circuit explained above is defined as an RCC type switching regulator.
In the RCC type switching regulator, when an output voltage value is Vout, input voltage value is Vin, the ON time of transistor Tr1 is Ton and the OFF time of transistor Tr1 is Toff, the relationship is defined by Vin×Ton=Vout×Toff. However, when the number of turns of the primary coil of the transformer T1 is assumed to be identical to the number of turns of the secondary coil, this formula can be modified so that Vout=(Vin×Ton)/Toff. Moreover, it can be modified by the period of ON/OFF of the transistor Tr1 replacing T, producing Vout=(Vin×Ton)/(T−Toff).
As indicated in the above formula, the input current can be adjusted by controlling the ON time of transistor Tr1 while the output voltage is kept constant. Thus, even when the load connected to the output terminal of the power supply varies, the value of Vout can be maintained constant using the feed-back control that controls the ON time of the transistor Tr1 by monitoring the output voltage Vout.
FIG. 2 is a schematic diagram of another power supply circuit of the related art. The circuit of FIG. 2 is different from the RCC type switching regulator in that the voltage converting and rectifying section 80 is formed by integrating the voltage converting section 20 and rectifying section 30 of FIG. 1. Rectifying section 80 is therefore provided in place of individually providing a voltage converting section and a rectifying section.
In FIG. 2, when the transistor Tr1 is ON, an input current flows through the primary side coil of the transformer T1. This causes the output current to flow through the secondary side coil of the transformer T1. This type of circuit is defined as a FORWARD type switching regulator.
In FIG. 2, the transformer T1 operates as a switch circuit. The transformer T1 does not operate as a voltage converting circuit. Therefore, a choke coil L2 and a flywheel diode DO are required for voltage conversion in addition to the transformer T1. In the circuit of FIG. 2, the relationship of the voltage to the time the transformer T1 is ON is Vout=(Vin×Ton)/(Ton+Toff)=(Vin×Ton)/T.
In addition, the current flowing through L2 also flows in the output detecting section 40 and the noise eliminating filter section 10 while the transistor Tr1 is ON. Moreover, current flowing through L2 is supplied via D1 while the transistor Tr1 is OFF. Therefore, an average input current Iin to the power supply circuit becomes equal to a product of an output current lout and the ON time of transistor Tr1. Accordingly, the relationship of current to the time transistor Tr1 is ON is Iin=(Iout×Ton)/T.
As indicated in the above formula, controlling the ON time of the transistor Tr1 can cover variation of the input voltage. Moreover, even when the capacity of the load connected to the output of the power supply is varied, Vout can be maintained constant by having the feedback control vary the ON time of the transistor Tr1 in accordance with the output voltage Vout.
FIG. 3 is a schematic diagram illustrating details of the sense circuit 70 and control circuit 60 that monitor the output power in the circuit illustrated in FIG. 1 or FIG. 2. The sense circuit 70 includes a voltage amplifier AMP 11, a couple of error amplifiers ERA 11, ERA 12 and reference voltage sources e11, e12. The control circuit 60 includes a triangular wave oscillator 66, a PWM comparator 62 and a drive circuit 68.
The reference voltage source ell is the reference voltage used to determine the output current value. The reference voltage source e12 is the reference voltage used to determine the output voltage value.
The voltage amplifier AMP11 measures a voltage drop generated by a current flowing through the sense resistor R0. The voltage amplifier AMP11 outputs a voltage that is proportional to a current value flowing through the sense resistor R0. The error amplifier ERA11 compares an output voltage value with the reference voltage value e11. The error amplifier ERA11 outputs a low level when a large current flows through the sense resistor RO or a high level when a small current flows through the sense resistor R0.
Similarly, the error amplifier ERA12 compares an output voltage value of the power supply with the reference voltage value e12. The error amplifier ERA12 outputs a low level when the power supply outputs a high output voltage value or a high level when the power supply outputs a low output voltage value.
The PWM comparator 62 is a voltage comparator including one inverting input and a plurality of non-inverting inputs. Namely, the PWM comparator 62 illustrated in FIG. 3 is a voltage pulse width converter for controlling the ON time of an output pulse depending on an input voltage value. The PWM comparator 62 compares the minimum voltage value among a plurality of non-inverting inputs shown by a +, with the voltage value of an inverting input shown by a −. The PWM comparator 62 provides an output when the voltage value of inverting input is lower. An output signal from the triangular wave oscillator 66 is inputted to the inverting input of the PWM comparator 62. Meanwhile, the output from the error amplifier ERA11 and the output from the ERA12 are inputted to the non-inverting input.
During the period where the triangular wave voltage value from the triangular wave oscillator 66 is lower than the output voltage of error amplifier ERA11 and is also lower than the output voltage value of the error amplifier ERA12, an output voltage from the PWM comparator 62 is inputted to the drive circuit 68. With this input, the drive circuit 68 is driven to drive the switching transistor Tr1 of the power supply. Moreover, during the period where the triangular wave voltage value from the triangular wave oscillator 66 is higher than the output voltage value of the error amplifier ERA11 or the triangular wave voltage value from the triangular wave oscillator 66 is higher than the output voltage value of the error amplifier ERA12, an output is not provided to the drive circuit 68 from the PWM comparator 62. Thereby, drive of the drive circuit 68 stops and the switching transistor Tr1 of the power supply turns OFF.
As explained above, the switching transistor Tr1 is turned OFF depending on the output voltage value of the power supply that is detected with the sense circuit 70. The power supply control circuit controls an output voltage and an output current of the power supply with the structure explained above.
In FIG. 3, an output from the sense circuit 70 for monitoring an output voltage and an output current is then inputted directly to the PWM circuit 62 of the control circuit 60. However, if electrical isolation is required between the sense circuit 70 and the control circuit 60, a photocoupler is connected to each input end of the PWM circuit 62. Such electrical isolation can also be realized by attaching a photocoupler to the output of the sense circuit 60.
In the circuit illustrated in FIG. 3, the PWM comparator 62 selects a lower voltage value among the outputs of the error amplifiers ERA11 and ERA12. However, it is also possible that the sensor circuit 60 combines the output voltages of the error amplifiers ERA11 and ERA12 and transmits only the lower voltage value.
FIG. 4 is a schematic diagram illustrating a circuit to realize such modification. The circuit illustrated here is an analog circuit to transmit the lower voltage value among the outputs of the error amplifiers ERA11 and ERA12. When a voltage of the error amplifier ERA11 becomes high, the base potential of the transistor Tr11 becomes high and a base current is reduced. Therefore, a collector resistance of Tr11 becomes high and a constant current is supplied to the collector of Tr1 from the constant current source i. Therefore, the collector voltage of Tr11 becomes higher as the collector resistance of Tr11 becomes large.
When the voltage of error amplifier ERA11 becomes low, a base potential of Tr11 becomes low. Therefore, since a base current increases, a collector resistance of Tr11 becomes small. Since a constant current is supplied to the collector of Tr11 from the constant current source i, a collector voltage of Tr11 becomes low in proportion to reduction of the collector resistor of Tr11.
Similarly, when a voltage of the error amplifier ERA 12 becomes high, a base potential of Tr12 becomes high. Thereby, since a base current is reduced, a collector resistance of Tr12 becomes large. A constant current is supplied to the collector of Tr12 from the constant current source i. Accordingly, a collector voltage of Tr12 becomes high in proportion to increase of collector resistance of Tr12.
When a voltage of the error amplifier ERA12 becomes lower, a base potential of Tr12 becomes low. If so, since a base current increases, a collector resistance of Tr12 becomes small. Since a constant current is supplied to the collector of Tr12 from the constant current source i, a collector voltage of Tr12 becomes low in proportion to reduction of collector resistance of Tr12.
The collectors of Tr11 and Tr12 are connected to the common constant current source i. Therefore, the collector voltage of Tr11 and Tr12 is fixed to the lower voltage. Accordingly, a lower voltage of the output voltage of error amplifier ERA11 or the output voltage of error amplifier ERA12 is outputted as the collector voltage of Tr11 and Tr12.
FIG. 7 is a structural diagram illustrating a related art external power supply that is connected to an electronic device with the power supply. In this figure, a docking station 130 is connected as a new load between the external power supply 100 and electronic device 110.
In FIG. 7, the power inputted from the external power supply 100 is supplied as the power source of the electronic device 110. Moreover, when a secondary battery 111 is provided within the electronic device 110, such power is also supplied as the charging power of the secondary battery 111. The power supplied to the electronic device 110 is converted to the voltage value required by the device with a voltage converting circuit 112.
The secondary battery 111 is a built-in battery to supply the power to the electronic device 110 when the power source from the external power supply 110 stops. Power supply 150 applies the power to charge the secondary battery 111 built in the electronic device. A microcomputer 113 detects start and end of charging at the time of charging the secondary battery 111. The microcomputer 113 controls the ON/OFF condition of the power supply from a charger and also controls the power supply.
Voltage comparator COMP101 detects that the power is supplied from the external power supply. COMP101 sends a high level to the microcomputer when a voltage value measured with voltage dividing resistors R103 and R104 is higher than reference voltage value e1. AMP101 measures a current value supplied to the secondary battery 111 from the power supply 150. A diode D101 prevents leak of power of the secondary battery 111 to the external circuit. A diode D102 prevents direct application of the power from the external power supply to the secondary battery 111 when the power is supplied from the external power supply.
The power supply 150 is a DC-DC converter to generate a constant voltage or a constant current or a constant voltage current. The power supply 150 includes a sense resistor R101 for measuring an input current value from the external power supply, a sense resistor R102 for measuring a charting current value of the secondary battery 111, a main switch transistor FET101, a choke coil L101, a flywheel diode D103, a smoothing capacitor C101 and a control circuit. Detailed operation of the power supply 150 is not explained here, because it is similar to the operation explained above.
The docking station 130 is provided to expand the functions of the electronic device 110. This docking station 130 can expand the functions of the electronic device through connection with the electronic device. Portability is a very important factor in the portable type information device, such as a notebook size personal computer.
Therefore, the basic section of the notebook size personal computer must be given the required minimum functions through reduction in size and weight as much as possible. For this reason, the LAN connecting mechanism which cannot be used during the transportation and CD drive or DVD drive which is not used frequently are no longer loaded to the body of notebook size personal computer. Meanwhile, it is very convenient when various functions are provided in the notebook size personal computer for when it is used on the desk. Therefore, the docking station is provided with high level functions. Connecting a notebook size personal computer to the docking station can attain expandability for use of various devices.
In FIG. 7, the external power supply 100 sends the power to both electronic device 110 and docking station 130. When the secondary battery 111 is charged in the side of electronic device 110, the power supply 150 tries to extract the maximum current specified at the time of design from the external power supply 100. However, the power of external power supply 100 is also supplied to the docking station 130. Accordingly, the external power supply 100 enters the over-load condition and thereby shuts off the output.
Therefore, the external power supply 100 for docking station 130 shall have the capacity to cover the addition of the maximum current used for the docking station 130 to the power consumption of the electronic device 110.
However, in this case, if a load current, for example, of the docking station 130 is reduced depending on the operating condition, an extra power cannot be used as the charging power of the secondary battery in the side of electronic device 110.
As a method of overcoming such problem, there is proposed a charging control circuit (Japanese Published Unexamined Patent Application No. HEI 10-286586) for keeping constant the output voltage of external power supply and controlling the charging current by monitoring an output voltage value of the external power supply. However, this circuit cannot attain the result as expected when an output power value of external power supply does not conform to the theoretical value.